This invention relates generally to thermally protective layers and, more particularly, to a thermal protective layer suitable for protection of a missile or air frame casing structure against a high intensity laser weapon.
As is known in the art, recent developments in laser technology have provided high intensity laser systems which make missiles, aircraft, spacecraft, and other structures vulnerable to laser attack. Accordingly, it is necessary to provide a system for making substantially invulnerable the outer structure or casing of existing and future missile, aircraft, spacecraft or the like, thereby protecting critical components of the system including propulsion and guidance systems, fuel and payload.
It is known in the art that to protect a structure from a high energy weapon such as a laser, an ablating layer is generally provided to dissipate the energy of the laser. Ablation is a known phenomenon by which energy incident upon an ablating material is dissipated through vaporization of the material rather than by conversion of the energy into heat. That is, during exposure to the laser beam, the material of the ablating layer is vaporized away dissipating the laser energy by converting the solid material of the ablative layer into a vapor. However, ablation is but one phenomena which occurs during exposure of an ablating layer to a laser weapon. It is also known that some heat is generated in the ablating layer which can conduct through the ablating layer towards the structure to be protected. Typically, existing missile casings for example, are fabricated of light weight metals, such as aluminum. Aluminum becomes mechanically weakened at temperatures in excess of about 550.degree. K. Accordingly, conduction of thermal energy through the ablating layer to a missile casing comprised of such a material as aluminum will cause a build-up of heat at the missile casing which will mechanically weaken the missile casing.
A second problem caused by the heat generated during ablation of the outer layer is that the generated heat causes releasing or outgassing of gaseous components from the materials used to attach the ablating layer to the missile casing. Outgassing of these components can cause bond failures between the ablating layer and the missile casing, and thereby hastening the effect of the laser upon the missile.
Several solutions to some of these problems have been described in the art. One solution, for example, as described in U.S. Pat. No. 4,431,697 for a composite missile casing includes a pair of thermally protective layers comprising a cork composition layer, such as a cork phenolic which may in alternate configurations comprise carbon phenolic, silica phenolic, carbon nitrile or epoxy novalac coatings. Sandwiched between the pair of thermally protective layers is an adhesively bonded laser hardening barrier. The laser hardening barrier comprises a heavy metal bearing resin impregnated carbon fabric. One problem with the abovementioned solution is the requirement of having three composite layers. A second problem is that the intermediate layer comprising the heavy metal bearing resin impregnated carbon fabric does not have the mechanical strength, particularly interply tensile strength, of the pair of cork phenolic layers, in particular, the silica phenolic layer. That is, the heavy metal bearing resin impregnated carbon fabric may have insufficient tensile strength to carry the load of the outer cork phenolic layer attached thereto. This could aid in causing bond failure between the cork phenolic outer layer and the laser barrier layer.
A further problem with the above-described solution is that heating of the outer cork composition layer may cause a build-up of outgaseous products at the bonding interface between the cork insulating layers and the laser hardened barrier. Further, the use of a heavy metal such as tungsten adds weight to the layer and hence the missile and further is expensive.